Portable breathing systems are used to enable their user's to function in an environment which lacks oxygen or in an environment containing substances which would be toxic if inhaled. For example, they are used in industrial plants where toxic chemicals have been spilled, or where there is a fire, e.g. in a mine. Various breathing systems are known, these systems can be divided into three broad categories.
A first category are those systems which provide a breathable gas to the user and in which the user's exhalate is exhausted out of the system i.e. the system is open. Such systems typically use compressed air or a compressed blend of oxygen and nitrogen. Such compressed air systems are advantageous both in cost and weight in circumstances where a separate compressed air supply not carried by the user is readily available and where length of a supply hose is not a limiting factor. Compressed air systems have a weight disadvantage over the other systems when the compressed air source of the system is made portable. As all of the consumed air in a compressed air system is exhaled from the system, these systems have the highest gas consumption for a given operating duration.
Another type of breather system is such as described in U.S. Pat. No. 3,794,030 entitled "EMERGENCY BREATHING APPARATUS" which issued Feb. 26th, 1974. In this type of breathing system, exhaled gas rather than being discharged from the system is passed through a chemical bed of, for example, potassium superoxide and then released to the user, i.e. the system is closed. The superoxide reacts with the exhaled gas to remove carbon dioxide therefrom and at the same time to release oxygen which will mix with the exhaled gas to revitalize the exhaled gas for rebreathing. A disadvantage with this type of system is that considerable heat is generated by the chemical reaction.
A third general category of breathing systems are those in which the exhaled gas is treated by removing carbon dioxide from it and adding oxygen to it to replenish the oxygen consumed by the user. Again, this is a closed system. A problem with this type of a system is to maintain a relatively constant concentration of oxygen. If too much oxygen is added to such a system, the system eventually becomes oxygen rich and any gas leakage from around the connection between the system and the user's face, or elsewhere, could be quite hazardous in a combustible environment. If not enough oxygen is added to the rebreathed gas, the user will, of course, suffer from oxygen shortage. It is possible to use an oxygen probe to monitor the oxygen concentration in the system and thereby electronically control the amount of oxygen added to the system. Disadvantages with such an electronic oxygen control system are the attendant cost and as well the increased likelihood of failure. Adding an electrical system to what would otherwise be a purely mechanical system introduces another system along with its attendant risk of failure.